1. Field of the Invention
The present invention relates to a process for forming an insulating thin film on a substrate by sputtering, and a metallic substrate coated with an insulating thin film formed by said process.
2. Description of the Related Art
In the field of electronics, there is a strong demand for a reduction of the size, an increase of the speed, and an increase of the functions of elements and devices, and to develop an insulating metal substrate capable of meeting this demand, an insulating thin film to be formed on a metallic substrate is currently produced by the following four processes.
(1) PVD processes such as vacuum evaporation wherein a source material is heated and evaporated under a low pressure to form a thin film, and ion plating and sputtering wherein a physical action by bombardment of ions or electrons in a gaseous phase is utilized to form a thin film. PA1 (2) A CVD process wherein various energies are applied to a gas for activation and excited species are reacted with each other to deposit a new solid species on a substrate. PA1 (3) A process wherein the surface of a substrate is oxidized or nitrided by a gas species in a heated or plasma state. PA1 (4) A process wherein an anodic formation is conducted in an aqueous solution.
Recently, among the above-described processes for the formation of an insulating thin film, ion plating and sputtering (particularly magnetron sputtering) have been used due to the advantage thereof of an elimination of the necessity to heat the substrate and a lower susceptibility to the inclusion of impurities. In the above-described processes, however, the substrate is bombarded by electrons, ions or high energy neutral particles, and consequent damage to a thin film in an early stage of the growth thereof causes a growth of film containing voids, so that through-holes penetrating the substrate and leading to the surface of the thin film are formed: This unfavorably causes an increase in the leak current and a lowering in the dielectric strength.
In general, if it is assumed that the defect density n causing a leak current and a breakdown of an insulating property when a voltage is applied across the electrode and the substrate, follows the Poisson distribution, then the following relationship is established: EQU Z=exp {-n.S} (1)
wherein Z is a proportion at which the insulating property is ensured (percentage insulation) and S is an electrode area. Further, the equation (1) can be rewritten as: EQU log(Z)=-n.S (2)
That is, as the electrode area increases, the percentage insulation is lowered exponentially. FIG. 1 shows a relationship between S (electrode area) and log (Z) (logarithm of a proportion at which the insulating property of a resistance of 20 M.OMEGA. or more is maintained at an applied voltage of 1 V where a 0.5 .mu.m-thick Al.sub.2 O.sub.3 film has been formed on a stainless steel substrate by magnetron sputtering). As apparent from the drawing, it is difficult to ensure the insulating property when the electrode area is 20 mm.sup.2, and thus to fill up the pin holes, it is necessary to increase the thickness of the film (see, for example, M. N. KHAN: Thin Solid Films, 124 (1985) 55-58), i.e., the thickness must be increased to 1 .mu.m or more in the case of the magnetron sputtering process and 2 .mu.m or more in the case of the ion plating process.
To avoid the bombardment of the substrate by electrons, ions and high energy neutral particles during the growth of the thin film, an attempt has been made to provide an opposed target sputtering process wherein a pair of targets are provided opposite to each other, a magnetic field is applied between the targets to confine a plasma in the space, and a substrate is provided outside the plasma to form a thin film (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 63-121658).
The bombardment of the substrate by the ions or high energy neutral particles, however, serves to repair the defect and increase the density of the thin film. When a thin film is formed on a metallic or other substrate having a rough surface, the existence of a portion of the substrate on which a thin film material is not deposited, due to a shadow effect, causes the defect to remain unfilled if the above-described bombardment action is not utilized, and thus a thin film having a low insulating property is unfavorably formed.
An object of the present invention is to solve the above-described problem through the use of sputtering in two different atmospheres or conditions in the formation of an insulating thin film on the surface of a substrate, and to provide a process for the formation of an insulating thin film exhibiting an excellent insulating property even when the thickness is 1 .mu.m or less.